Well respected researchers in the field of amorphous silicon alloy materials espouse the theory that an inherent limitation exists as to the rate at which high quality amorphous silicon alloy material can be grown. As used herein, high quality amorphous silicon material will be defined as material which exhibits a low density of electronic defect states in the forbidden band gap thereof so as to provide for good electrical characteristics (such as high photoconductivity, low dark conductivity ability to be doped by the addition of impurity atoms).
In 1987, Tsai, Shaw, Wacker and Knights of Xerox PARC published a paper entitled "Film Growth Mechanisms Of Amorphous Silicon In Diode And Triode Glow Discharge Systems" in Materials Research Society Symposium Proceedings. Vol. 95, p. 219. These researchers drew a distinction between the deposition of amorphous silicon by (a) chemical vapor deposition (CVD) processes, which processes are rate limited by a surface reaction causing conformal step coverage and are produced by low sticking coefficient plasma deposition species such as SiH.sub.3 ; and (b) physical vapor deposition (PVD) processes, which processes are not surface reaction rate limited and are produced by high sticking coefficient plasma deposition species such as SiH.sub.2 and SiH. PVD processes result in poor step coverage and strong columnar morphology, and, the authors argue, give rise to films with high density of electronic defects.
The Xerox PARC paper found the deposition of high quality, non-columnar amorphous silicon alloy material, would, therefore, occur at low rates of deposition utilizing CVD-type processes, while low quality, strongly columnar amorphous silicon alloy material would occur at high rates of deposition utilizing PVD-type processes. The authors showed that by moving the substrate farther away from the plasma (using a triode deposition apparatus), species with longer lifetimes (lower sticking coefficients) are preferentially deposited because more of the high sticking coefficient species are eliminated by gas phase collisions. SiH.sub.3 free radical species had been earlier known to be either the dominant free radical or the primary mass transporting species under plasma conditions tailored for the deposition of device quality amorphous silicon. Since the effective sticking coefficient is defined as the probability that the mass transporting species contribute to the film growth, it represents the net effect of complicated reactions involved in the deposition of amorphous silicon films (such as incorporation, dissociation, re-emission, etching, hydrogen elimination, etc.). Xerox PARC thus conclude that microscopically different mass transporting species are associated with PVD and CVD processes; SiH.sub.2 and SiH species dominate PVD growth while SiH.sub.3 species dominate CVD growth. They state: ". . . There seems to be a limitation as to how fast one can grow good quality a-Si:H by plasma deposition[.]" and further ". . . [t]here seems to be an incompitability between good material quality and fast growth rate in the plasma deposition of amorphous silicon . . . "
The inventors of the subject matter disclosed herein present results which demonstrate that this conclusion reached by the Xerox PARC group, as well as other researchers in the field is erroneous. Specifically, the instant inventors demonstrate that no fundamental incompatibility exists between the high rate of deposition of amorphous silicon alloy material and the high quality of that material. As a matter of fact, the instant inventors will present experimental proof that, by utilizing the novel processing techniques described herein, the electronic quality of the depositing amorphous silicon alloy material actually increase with a corresponding increase in the rate of deposition thereof under some conditions.